For the creator:
- requires frequent repair checks (prototypes are usually finicky)
For everyone else:
- increase the misfire chance
- nonproficiency penalty
- longer loading times
To actually invent the thing? Lots of intelligence checks.
How would you guys handle letting a player invent someting? Specifficaly an advanced gun when you don't want the general game world to have access to it.
There's three options.
1) You don't. Advanced guns in the "Emerging Gun" settings specifically are roughly as rare as artifacts, and artifacts aren't craftable.2) You assign a ridiculous DC, and provide some way that that DC will eventually be reachable. For example: a DC 50 Craft (gunsmithing) check, where each check takes a year, and on each failure you can roll a raw Int check (DC 20 or so) to gain a +2 bonus towards all future checks.
3) You just say "Sure, you spend three years and here you go." You might also want to consider letting your wizards research epic level spells at 10th level, though.
If you and your players like playing the skills game: Knowledge Engineering checks to come up with each component that is different/advanced compared to the old.
So cartridged bullets, rifling, wheel-lock compartments (if you are going for a revolver), etc.
Then have the first craft check fairly high to make each part, and a final craft check to put them all together.
Have this happen over time span. So allowing one check per day at most on the knowlege checks, and assigned component values for each part being built giving the craft times.
...
Or, just hand-waive and say "you did it!", with possibly a craft check to build it.
It really all depends on what kind of gameplay you and your players find fun.